1. Field of the Invention
The present invention relates to a system for estimating the mechanical properties of materials, e.g. for estimating stress and stiffness in cardiac muscle, with an indentation probe.
2. Description of the Related Art
The structural elements of animal and human bodies are made up of a wide variety of soft tissues, semi-hard cartilage, and hard bones. The structural integrity of these elements is crucial to the proper operation of the body. Unfortunately, analyzing these elements in a living body can be difficult due to limitations on isolation and analysis. Most diseases, such as heart disorders, are diagnosed by some method other than direct tissue examination.
The performance of an intact heart is manifested by its global generation of pressure and flow. This performance is the integrated effect of many factors including the geometry of the chamber, the mechanical properties of each region of the wall, the load imposed on each portion of the wall, and coronary blood flow. Global measurements, however, only describe overall cardiac performance.
There are difficulties in inferring regional changes, e.g. diseased heart soft tissue, from global measurements because there may not be a one-to-one relation between the regional function and the global measurements. Soft tissue abnormalities in one portion of the heart may be compensated for or overshadowed by load changes in another. Regional mechanics must be measured to differentiate whether regional dysfunction, myocardial infarct expansion, cardiomyopathy progression, and arrhythmia generation are due to changes in regional loading or to changes in regional mechanical properties.
Quantifying the regional properties of soft tissue, cartilage, or bone requires the measurement of regional wall strain and stress. In heart soft tissue, regional wall strain has been measured reasonably accurately in vivo [1-3]. The regional wall stress, however, is more difficult to quantify accurately.
Ascribing a value to the stress and stiffness of various areas in a heart is critical for determining the effects of regional stress loading and contractility to proper ventricular functioning. Without accurate measurements of stress, it is difficult to determine whether abnormal pump function is due to abnormal muscle that cannot generate stress or to normal muscle that is generating stress but is abnormally loaded. The ability to differentiate between these possibilities would have important diagnostic ramifications. This ability is hampered by the available measuring systems, however.
Strain gauge devices have been used to measure wall stress [4,5]. These devices give uncertain results because of the unknown degree of coupling between the transducer and ventricular wall [6,7]. Mathematical models have also been used to predict stress [8-10]. However, these models cannot be validated because there are no reliable actual measurements of wall stress for comparison [6].
A promising new approach has looked at the relationship between indentation stress and strain. It has been shown in the isolated ventricular septa that the ratio of indentation stress to indentation strain was proportional to the in-plane wall stress during steady-state indentations [11].
The apparatus used to measure the stress/strain relationship in the isolated ventricular septa consisted of two parts. The first component was a biaxial, servo-controlled system that allowed independent control of force and length in two orthogonal directions (x and y axes) in the plane of the septa. In-plane forces were measured by transducers connected to thread carriages supporting the septa.
The second component was the transverse indenter. The indenter included an arm along the z-axis that was mounted to allow visual detection of positional markers. The probe was about 7 mm in diameter. A z-axis force transducer was positioned on the arm. A stepper-motor was calibrated for measuring the z-axis displacement of the probe surface indenting the septa. As the probe indented the septum, the resulting force was measured by the z-axis force transducer. In-plane forces were measured by the in-plane force transducers.
Measurements from the indentation probe were taken at a variety of depths over a series of beats. At least 20 stable contractions were required to determine the transverse stiffness of the specimen. The transverse stiffness could be determined only every 20-40 seconds because the indenting probe had to be accurately positioned at 6-10 different depths in the muscle and maintained in a constant indenting angle at each depth for 2-3 cycles. This delay between successive readings makes it difficult to follow changes in wall stress over time. It was not possible to follow changes in the wall stress over a single contraction. Due to the relatively long measuring period, this form of measurement is best described as "steady state".
It would, therefore, be desirable to provide a device that could determine the transverse stiffness in a short enough period of time to allow estimation of wall stress throughout a cardiac contraction cycle. Preferably, such a device could make the transverse stiffness determinations from either the endocardial or epicardial surfaces of the heart and could be used percutaneously.